This invention relates generally to gas turbine engines and more particularly, to multi-spool gas turbine engines.
At least one known gas turbine engine includes, in serial flow arrangement, a forward fan assembly, a core driven fan assembly, a high-pressure compressor for compressing air flowing through the engine, a combustor for mixing fuel with the compressed air such that the mixture may be ignited, a high pressure turbine for providing power to the high pressure compressor, and a low pressure turbine for providing power to the fan assembly. The high-pressure compressor, combustor, and high-pressure turbine are sometimes collectively referred to as the core engine. In operation, the core engine generates combustion gases, which are discharged downstream to an intermediate and/or low pressure turbine that extracts energy therefrom for powering the forward fan assembly.
To increase engine efficiency, at least one known turbofan assembly includes a counter-rotating low-pressure turbine and an intermediate-pressure turbine that are each coupled to a counter-rotating fan assembly. More specifically, to assemble a turbofan engine assembly that includes counter-rotating low-pressure and intermediate-pressure turbines, an outer rotating spool, a rotating frame, and two concentric shafts, are installed within the turbofan engine assembly to support the counter-rotating low-pressure and intermediate-pressure turbines. However, the use of a counter-rotating low-pressure turbine increases the overall engine weight, design complexity, and/or manufacturing costs of such an engine are increased. Other known turbofan assemblies have attempted to increase the rotor speeds and/or use higher pressure ratio fans. However, such modifications have provided only limited benefits, as the addition of more fan stages to increase pressure ratios also increases the overall engine weight and costs.